Question: $ D = \left[\begin{array}{rrr}0 & 5 & 2 \\ 5 & 0 & 3\end{array}\right]$ $ C = \left[\begin{array}{rr}1 & -2 \\ 4 & 4 \\ 2 & -1\end{array}\right]$ What is $ D C$ ?
Explanation: Because $ D$ has dimensions $(2\times3)$ and $ C$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ D C = \left[\begin{array}{rrr}{0} & {5} & {2} \\ {5} & {0} & {3}\end{array}\right] \left[\begin{array}{rr}{1} & \color{#DF0030}{-2} \\ {4} & \color{#DF0030}{4} \\ {2} & \color{#DF0030}{-1}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ C$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ C$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ C$ , and so on. Add the products together. $ \left[\begin{array}{rr}{0}\cdot{1}+{5}\cdot{4}+{2}\cdot{2} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ C$ and add the products together. $ \left[\begin{array}{rr}{0}\cdot{1}+{5}\cdot{4}+{2}\cdot{2} & ? \\ {5}\cdot{1}+{0}\cdot{4}+{3}\cdot{2} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ D$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ C$ and add the products together. $ \left[\begin{array}{rr}{0}\cdot{1}+{5}\cdot{4}+{2}\cdot{2} & {0}\cdot\color{#DF0030}{-2}+{5}\cdot\color{#DF0030}{4}+{2}\cdot\color{#DF0030}{-1} \\ {5}\cdot{1}+{0}\cdot{4}+{3}\cdot{2} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{0}\cdot{1}+{5}\cdot{4}+{2}\cdot{2} & {0}\cdot\color{#DF0030}{-2}+{5}\cdot\color{#DF0030}{4}+{2}\cdot\color{#DF0030}{-1} \\ {5}\cdot{1}+{0}\cdot{4}+{3}\cdot{2} & {5}\cdot\color{#DF0030}{-2}+{0}\cdot\color{#DF0030}{4}+{3}\cdot\color{#DF0030}{-1}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}24 & 18 \\ 11 & -13\end{array}\right] $